Transformer Matching

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Mondial85

Mondial85

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Joined 2015
#1
Hey guys, I know I probably shouldn't be here since I'm really a novice when it comes to electronics but I have a question that I'm sure doesn't have a simple answer. Ya'll don't laugh now, please. And I really don't think I'm stupid, but I'll let you be the judge of that. When building a vacuum tube amplifier, selecting the output tube and the output transformer; am I trying to match the plate resistance of the output tube to the impedance on the primary of the output transformer? For instance: If my tube (let's say a triode) has a plate resitance of 7,000 ohms, would I be looking for an output transformer who's primary is as close to 7,000 ohms as I can? I know, I know, it's not that simple and many more factors also must be considered, but is that maybe a starting point?
Thanks.
 
#2
that is a pretty high plate resistance for a triode unless it is a small signal triode,
but anyway my rule of thumb for triode is two times plate resistance for maximum power, 3 times or more for lower distortion....

as an example, the 300b has a plate resistance of 800 ohms, so that 3000 ohm primary opt's are often used....
 
#3
Thanks. I didn't really have an OPT in mind, I was just more or less using that as an example. In many of the designs I've been seeing, it appears that plate to plate load resistance seemed to be a factor, too. It seems that the P2P load resistance of the tubes are pretty close to the OPTs primary. Is that a factor, also?
 
#4
No, you are not trying to match the anode/plate resistance to the resistance presented by the OPT primary. You are trying to get the best compromise between power output and distortion. Some datasheets or application notes will give graphs of this.

For a triode output, the presented load will typically be a bit higher than the anode resistance. For a pentode output, the presented load will be much smaller than the anode resistance.
 
#5
Mondial85 said:
Thanks. I didn't really have an OPT in mind, I was just more or less using that as an example. In many of the designs I've been seeing, it appears that plate to plate load resistance seemed to be a factor, too. It seems that the P2P load resistance of the tubes are pretty close to the OPTs primary. Is that a factor, also?
Click to expand...

by P2P you are talking push-pull, thought your original question was for single ended...now you lost me....😉
 
#7
OK, so it appears there's no general "rule of thumb". I didn't know. I'm currently building a pair of mono-block power amps. Each will use a pair of WE 349As in push-pull driven by a single WE 310A. I really didn't know what the primary should be on the OPT.
Thanks
 
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